Production of benzene dicarboxylic acids



United States Patet PRODUCTION OF BENZENE DICARBOXYLIC ACIDS WalterSpeer, Schwetzingen, Germany, assignor to Badische Anilin- & Soda-FabrikAktiengesellschaft, Ludwigshafen, Rhine, Germany No Drawing. ApplicationDecember 6, 1957 Serial No. 700,979

Claims priority, application Germany December 24, 1956 9 Claims. (Cl.260-524) The present invention relates to an improved process for theproduction of benzene dicarboxylic acids by oxidation of xylenes withoxygen or gases containing the same. In particular it relates to theproduction of benzene dicarboxylic acids, above all terephthalic acid,by oxidation of xylenes with oxygen, preferably in the presence of theusual oxidation catalysts, with the simultaneous co-employment of anadditional substance promoting oxidation not hitherto used.

Among the aromatic dicarboxylic acids, terephthalic acid especially hasgained importance in recent years as an intermediate product for theproduction of plastics and synthetic fibres. Various methods are knownfor its production. For example it may be obtained by rearrangement ofpotassium salts of other benzene dicarboxylic acids or by oxidation ofdialkyl-substituted benzenes. Carrying out the oxidation by the use ofoxygen or gases containing oxygen as oxidizing agent is however attendedby difiiculty because the oxidation comes to a standstill at a certainincomplete conversion which depends on the dialkylbenzene used and thencannot be carried appreciably further even by intensifying theconditions of the reaction. Thus it is known that the oxidation ofpara-xylene itself with the use of catalysts practically proceeds onlyas far as the para-toluic acid stage because this can only be furtheroxidized with great diflfiiculty. Carrying out this process on atechnical scale is therefore hardly possible, unless it be byesterifying the toluic acid formed and further oxidizing the ester.

It is an object of this invention to efiect, by co-employment of anadditive, the activation of the oxidation of xylene with oxygen, air orother gaseous oxygen-containing oxidizing agent, if desired in thepresence of the usual oxidation catalyst, so that both alkyl groups ofthe xylene to be oxidized are substantially converted in a single stageinto the carboxylic acid.

Another object of the invention is to facilitate the oxidation ofdialkyl benzenes by reaction-promoting substances so that it leads morereadily over the stage of the monoalkylbenzene carboxylic acid and theintermediate oxidation products to an increased formation of benzenedicarboxylic acids.

A further object of the invention is therefore to obtain directly higheryields of benzene dicarboxylic acids by the oxidation of dialkylbenzeneswith oxygen, air or other oxygen-containing gases in one oxidationstage.

I have found that this and other objects, such as may be seen in thefollowing description, can be realised and good yields of benzenedicarboxylic acids, especially terephthalic acid obtained in a simpleand economical way by carrying out the oxidation of a xylene withoxygen, air or another oxygen-containing gas in the liquid phase attemperatures of 120 to 200 C., if desired in the presence of one of theusual oxidation catalysts, in the presence of a lower saturatedaliphatic alcohol. Of these, methanol is the most suitable alcohol forthe process.

As the initial xylene there may be used the isomeric xylenes as such oralso in admixture with each other. The xylene or mixture of xylenes mayalso contain small amounts of other hydrocarbons, for example benzene,toluene and ethylbenzene; for example it is possible to start frommixtures such as are present in crude xylene or of a technical gradexylene.

As alcohols in the presence of which the oxidation is carried outaccording to this invention, there come into question the lower membersof the saturated aliphatic series, especially methanol, and alsoethanol. The alcohols are added in small amounts to the oxidationmixture, for example amounts of 0.5 to 5% by weight per hour withreference to the weight of the xylene to be oxidized. When usingmethanol, good results are in general obtained when the methanol isuniformly dispersed in an amount of about 1 to 2% by weight per hourwith reference to the amount of hydrocarbon being oxidized. Since theamount of catalyst employed as well as the amount of hydrocarbons otherthan xylene is small, the amount of methane added hourly can beapproximately calculated with respect to the initial oxidation mixtureof hydrocarbon and catalyst and will fall within the above-mentionedranges. While the introduction of the alcohol may be begun with beforethe oxidation, it is the preferred practice of my invention to start theaddition of the alcohol only when the absorption of oxygen is already inprogress.

The oxidation takes place in the usual way under the conditions knownfor working in the liquid phase. For example the procedure may be thatthe initial material, to which an oxidation catalyst has preferably beenadded, is heated to a temperature of about to 200 C., advantageously toC. and leading in oxygen, air or other oxygen-containing gas in such anamount that the oxygen or the oxygen component of the gas is absorbedwholly or partly, for example to the extent of about half. As soon as agood absorption of oxygen has been achieved and consequently theoxidation has appreciably commenced, the addition of alcohol is begun.The alcohol may be added either together with the oxygen or air or awayfrom the oxygen or oxygen-containing gas. The introduction of thealcohol into the oxidation mixture should be distributed uniformly overthe entire reaction time. In order to avoid an inordinately strongvaporization of the initial material, the oxidation may if desired becarried out under increased pressure, for example at 2 to 30, morepreferably 10 to 20, atmospheres. Working under increased pressure isalso especially of advantage when air or other oxygen-containing gas isused as the oxidizing agent. When Working under atmospheric pressure theuse of oxygen is to be preferred to the use of air.

Although the oxidation already proceeds in a satisfactory way to theoxidation of the desired benzoic dicarboxylic acid even without anoxidation catalyst, and considerable amounts of dicarboxylic acid areformed, it is preferable to co-employ such reaction-acceleratingsubstances.

As catalysts which may be dissolved or dispersed in the initial materialthere come into question oxidation catalysts which are known per se. Forexample there may be mentioned the salts of organic higher carboxylicacids, as for example of ethylhexane acid, first running fatty acids ofthe oxidation of parafiins or naphthenic acids, of which the cation isformed by a metal which is capable of occurring in more than one valencystage, for example cobalt, manganese, cerium and copper. I

prefer the cobalt salts. The amounts in which the cata-.

lysts may advantageously be co-employed lie in general between 0.01 and1% by weight with reference to the hydrocarbon to be oxidized.

The gases leaving the oxidation zone are preferably cooled and thecondensate which separates is returned to the oxidation vessel afterseparation of the water formed. The oxidation mixture contains as mainproduct the benzene dicarboxylic acids, besides unoxidized hydrocarbon.There are also contained therein methylbenzene monocarboxylic acids andother partial oxidation products. These intermediate oxidation prodnetsare however contained in considerably smaller amounts than in the prioroxidation methods which work without the presence of lower aliphaticalcohols in the oxidation of the dialkyl substituted benzenes. Theoxidation mixture obtained when working according to this inventioncontains the benzene carboxylic acids partly in the form of the esterscorresponding to the lower saturated aliphatic alcohol co-employed inthe oxidation.

The benzene dicarboxylic acids can be separated from the oxidationmixture in a manner known per se, as for example by hot filtration andWashing of the filter residue with solvents, such as toluene, xylene ormethanol or, after separation of unreacted hydrocarbon, byesterification of the whole oxidation product and separation of theester.

The process may be carried out either discontinuously or continuously.

The following examples will further illustrate this invention but theinvention is not restricted to these examples.

Example 1 450 grams of para-xylene and 0.9 gram of cobalt ethylhexanateare charged into an oxidation vessel which is provided with a stirrer, agas inlet pipe, a dropping funnel and a condenser which is associatedwith a Water separator of which an overflow pipe leads back into theoxidation vessel, and the mixture is heated to 130 C. while leading in15 litres of oxygen per hour. At the beginning the oxygen is absorbed inan amount of about litres per hour. After the temperature has risen to140 C., about 12 ccs. of methanol per hour are allowed to drip uniformlyinto the oxidation mixture. As the conversion proceeds, the reactiontemperature is raised to 170 C. and kept constant at this temperature.The gaseous and vaporous substances leaving the oxidation vessel arecooled in the condenser to a. temperature of about 15 C. so that thevaporous substances condense. The condensate passes into a waterseparator from which the aqueous phase containing methanol is run otffrom time to time while the unreacted xylene flows back into thereaction vessel. The uncondensed gaseous constituents, which contain theunabsorbed oxygen, escape to atmosphere at the top of the waterseparator. After about 40 hours the absorption of oxygen is finished.The reaction mixture, which after distilling off the unoxidized xylenehas an acid number of 330 and an ester number of 185, contains 359 gramsof terephthalic acid and 140 grams of toluic acid which are partlypresent already in combined form as the methyl ester and after completeesterification with methanol under increased pressure and at elevatedtemperature can be separated by distillation.

If the oxidation is carried out under the same conditions in the sameway but without the addition of methanol during the oxidation, there areobtained, after the absorption of oxygen is finished and working up hasbeen carried out in the manner described above, 78 grams of terephthalicacid and 299 grams of toluic acid in the form of the methyl ester.

Example 2 Into 500 grams of para-xylene in which 0.9 gram of cobaltethyl-hexanate have been dissolved there are led 15 litres of oxygen perhour at 125 C. in the Way described in Example 1 and at the same time 8ccs. of methanol are dripped into the reaction vessel per hour uniformlydistributed over the time. After 35 hours, the reaction is discontinuedand the contents of the reaction vessel, after distilling off theunreacted paraxylene, are esterified with 4 times the weight of methanolfor 6 hours at 240 C. After Working up the esterification mixture thereare obtained 146 grams of dimethyl terephthalate and 352 grams of methylparatoluate.

By Working in the same way and using the same oxidation mixture but withthe difference that the oxidation is carried out at the oxidationtemperatures set out in the following table, the following amounts ofoxidation products are obtained after esterification of the oxidationmixture in way described in the preceding paragraph:

423 grams of crude xylene obtainable in commerce, after the addition of1 gram of cobalt ethyl-hexanate and after heating the mixture to 140 C.are oxidized in the way described in Example 1 by leading in 25 litresof oxygen per hour; after the absorption of oxygen has evidentlystarted, 10 ccs. of methanol per hour are introduced in uniformdistribution over the whole of the 1 hour period. The temperature israised to 160 C. as the reaction proceeds and while leading in oxygenand adding methanol in the hourly amounts stated, and kept at the saidtemperature. After 35 hours the oxidation is discontinued. The oxidationmixture, after distilling off the unreacted crude xylene, is worked upby esterification of the oxidation product remaining as a residue, andseparating the ester. 224 grams of a mixture of dimethyl phthalates and204 grams of a mixture of the methyl esters of the isomeric toluic acidsare obtained.

Example 4 500 grams of meta-xylene are oxidized in the way described inExample 1 with oxygen with the simultaneous addition of 20 ccs. ofmethanol per hour. The reaction temperature is keptfirst at 140 C. for12 hours and then at 160 C. for 38 hours. By working up the oxidationmixture, after distilling off the unreacted xylene and esterifying theresidue, there are obtained 402 grams of dimethyl isophthalate and 151grams of methyl metatoluate.

Example 5 500 grams of para-xylene and 1 gram of cobalt ethyl hexanateare charged into the apparatus described in Example 1 and heated upwhile leading in 20 litres of oxygen per hour. As soon as the reactionhas commenced, which is the case between and C., 6 ccs. of methanol perhour are dripped uniformly into the reaction vessel. The temperature israised to 155 C. corresponding to the progress of the reaction and keptat this temperature. After 20 hours, the constituents which haveseparated in solid form in the reaction mass are filtered oil in asuction filter heated to C. by super heated steam. The filter residue iswashed with 200 grams of para-xylene heated to 130 C. The filtratecontaining xylene, the xylene used for washing and also the xylene whichis recovered by evaporation in vacuo from the filter cake are recoveredand further oxidized" under the said conditions. After 1,200 grams ofpara xylene have been used up in this way, there are obtained, afteresterification of the combined filter residues with methanol, a total of943 grams of dimethyl terephthalate and 240 grams of para-toluic acidester. The methyl paratoluate can be returned to the reaction vessel.

Example 6 5 kilograms of para-xylene and 10 grams of cobalt ethylhexanate are charged into a pressure-tight vessel of litres capacitywhich is provided with a rapidly rotating stirrer, a condenser and awater separator, and which has a separator which is connected to thereaction vessel through an overflow. The reaction vessel and also theoverflow arranged at a level of 10 litres and the separator are capableof being heated by a steam jacket. Reaction vessel, overflow andseparator are heated to 160 C. and air is led through the system under apressure of 15 atmospheres. 1,500 litres of residual gas are releasedfrom pressure per hour behind the condenser. When the reaction hascommenced, which can be ascertained by a diminution of the oxygencontent in the residual gas, a mixture of 220 grams of para-xylene, inwhich 0.1% of cobalt ethyl-hexanate is dissolved, and 100 grams ofmethanol is pumped in hourly by means of a cam shaft pump. The residencetime of the xylene present in the pressure vessel amounts to about 35hours under these conditions. From the separator, the hot reactionproduct which overflows is released from pressure in a vessel which isprovided with a condenser by which para-xylene distilled off iscondensed and pumped back into the reaction vessel by the cam shaftpump. The residue which becomes solid when cold is worked up byesterification with methanol as described in the foregoing examples.There are thus obtained in continuous operation from each 1,000 grams ofpara-xylene by crystallizing out an average of 900 grams of dimethylterephthalate and by distillation 310 grams of methyl para-toluate. Themethyl para-toluate is mixed with para-xylene and pumped back into thereaction vessel for further oxidation.

What I claim is:

1. In the process for the production of a benzene dicarboxylic acid bythe oxidation of a xylene in the liquid phase in an oxidation zone withan oxidizing gas containing oxygen at a temperature of between 120 and200 C. and in the presence of an oxidizing catalyst, the improvementwhich comprises carrying out the oxidation in the presence of a lowersaturated aliphatic alcohol introduced uniformly into the oxidation zoneat a rate of 0.5 to 5% by weight an hour, percentage with reference tothe amount of xylene to be oxidized.

2. The process as claimed in claim 1 wherein the lower saturated alcoholis methanol.

3. In the process for the production of a benzene dicarboxylic acid bythe oxidation of a xylene in the liquid phase in an oxidation zone withan oxidizing gas selected from the group consisting of oxygen and air ata temperature of between and 200 C. and in the presence of an oxidizingcatalyst, the improvement which comprises introducing into the oxidationzone a lower saturated aliphatic alcohol uniformly at a rate of 0.5 to5% by weight an hour, percentage with reference to the amount of thexylene to be oxidized, the lower saturated aliphatic alcohol beingintroduced after the absorption of the oxygen has commenced.

4. The process as claimed in claim 3 wherein the lower saturated alcoholis methanol.

5 The process as claimed in claim 3 wherein the xylene is a mixture oftechnical-grade xylene isomers.

6. In the process for the production of terephthalic acid by theoxidation of para-xylene in the liquid phase in an oxidation zone withoxygen at a temperature of between 120 and 200 C. and in the presence ofan oxidizing catalyst the improvement which comprises introducingmethanol uniformly into the oxidation zone at a rate of 0.5 to 5% byweight an hour, percentage with reference to the amount of thepara-xylene to be oxidized, the methanol being introduced after theabsorption of oxygen has commenced.

7. The process as claimed in claim 3 wherein the oxidation reaction iscarried out under a pressure of from 2 to 30 atmospheres.

8. The process as claimed in claim 4 wherein the oxidation reaction iscarried out under a pressure of from 10 to 20 atmospheres.

9. The process as claimed in claim 4 wherein the oxidation reaction iscarried out at a temperature of from C. to C.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN THE PROCESS FOR THE PRODUCTION OF A BENZENE DICARBOXYLIC ACID BYTHE OXIDIATION OF A XYLENE IN THE LIQUID PHASE IN AN OXIDATION ZONE WITHAN OXIDIZING GAS CONTAINING OXYGEN AT A TEMPERATURE OF BETWEEN 120* AND200* C. AND IN THE PRESENCE OF AN OXIDIZING CATALYST, THE IMPROVEMENTWHICH COMPRISES CARRYING OUT THE OXIDATION IN THE PRESENCE LOWERSATURATED ALIPHATIC ALCOHOL INTRODUCED UNIFORMLY INTO THE OXIDATION ZONEAT A RATE OF 0.5 TO 5% BY WEIGHT AN HOUR, PERCENTAGE WITH REFERENCE TOTHE AMOUNT OF XYLENE TO BE OXIDIZED.